I don't mean to butt in on a question for someone else,but I love Yellowstone and here is what I have found. A lot of the fish do just as you say and run up into the smaller tribs. Good luck catching them though. They get very skittish because the tribs are pretty shallow. I'm sure some run up into the canyon. I've had most of my luck on the Iron spring creek and I think the little firehole when it gets into late July and August.
Lynn